Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock'

James Bottomley
2015-09-28T14:26:57Z

On Mon, 2015-09-28 at 08:58 +0000, David Laight wrote:
> From: Rafael J. Wysocki
> > Sent: 27 September 2015 15:09
> ...
> > > > Say you have three adjacent fields in a structure, x, y, z, each one byte long.
> > > > Initially, all of them are equal to 0.
> > > >
> > > > CPU A writes 1 to x and CPU B writes 2 to y at the same time.
> > > >
> > > > What's the result?
> > >
> > > I think every CPU's cache architecure guarantees adjacent store
> > > integrity, even in the face of SMP, so it's x==1 and y==2. If you're
> > > thinking of old alpha SMP system where the lowest store width is 32 bits
> > > and thus you have to do RMW to update a byte, this was usually fixed by
> > > padding (assuming the structure is not packed). However, it was such a
> > > problem that even the later alpha chips had byte extensions.
>
> Does linux still support those old Alphas?
>
> The x86 cpus will also do 32bit wide rmw cycles for the 'bit' operations.

That's different: it's an atomic RMW operation. The problem with the
alpha was that the operation wasn't atomic (meaning that it can't be
interrupted and no intermediate output states are visible).

> > OK, thanks!
>
> You still have to ensure the compiler doesn't do wider rmw cycles.
> I believe the recent versions of gcc won't do wider accesses for volatile data.

I don't understand this comment. You seem to be implying gcc would do a
64 bit RMW for a 32 bit store ... that would be daft when a single
instruction exists to perform the operation on all architectures.

James